A fluid machine shown in FIG. 6 is a fluid machine proposed by the inventors of the present invention in the course of developing the present invention. The fluid machine has a pumping mode for compressing and discharging working fluid, and a motor mode for converting fluid pressure to kinetic energy and thereby to output mechanical energy. The fluid machine operates in the following manner.
[Pumping Mode]
In this operational mode, a movable scroll 102p of a pump-motor mechanism 100p is rotated by applying a rotational force to a shaft 109p, to thereby suck in and compress refrigerant.
More in detail, the shaft 109p is rotated while an electromagnetic valve 107p and an inlet port 106p are closed. An expansion-and-compressor device, namely the fluid machine, sucks in the refrigerant from a low pressure port 111p, compresses the same by working chambers 103p, pumps out pressurized refrigerant through a pump-out port 105p into a high pressure chamber 104p and discharges the pressurized refrigerant through a high pressure port 110p, in the same manner to a well known scroll type compressor.
There are two ways for applying the rotational force to the shaft 109a, according to one of which the expansion-and-compressor device is disconnected from an engine by cutting off power supply to an electromagnetic clutch 300p and the rotational force is applied from an electric rotating machine 200p to the shaft 109a, and according to the other of which the expansion-and-compressor device is operatively connected to the engine and driven by the rotational force from the engine.
[Motor Mode]
In this operational mode, the movable scroll 102p is rotated by introducing superheated steam of the refrigerant into the high pressure chamber 104p and expanding the superheated gas by the pump-motor mechanism 100p, while the expansion-and-compressor device is disconnected from the engine by means of the electromagnetic clutch 300p. The rotation of the movable scroll 102p is transmitted to the shaft 109p to obtain a mechanical output.
In the fluid machine explained above, it is necessary to air-tightly seal a gap between the shaft 109a and a housing 230p of the fluid machine, for example by a shaft seal device 333 such as a lip seal, because the shaft 109a extends from the outside into the inside of the housing 230p. 
Since the air-tightness at the gap between the shaft 109a and the housing 230p is obtained by the shaft seal device 333, in which the shaft seal device 333 is pressed against an outer peripheral surface of the shaft 109a at a certain contact pressure, an energy loss will be generated due to friction resistance at the shaft seal device 333 when the shaft 109a is rotated.
Accordingly, an efficiency of energy collection will be decreased in the above described fluid machine, because the energy to be collected is decreased in the motor mode operation.
The maximum energy to be collected in the motor mode operation is smaller in comparison with collected heat quantity, as seen from p-h diagram shown in FIG. 7. Accordingly, the energy loss at the shaft seal device 333 for the energy collection occupies a relatively large portion of the maximum energy to be collected.